When transmitting an information signal, a spread spectrum reception apparatus which performs communication using DS-CDMA (Direct Sequence CDMA) system operates as follows. That is, it applies a first modulation such as QPSK to the information signal and thereafter spread-modulates the information signal with a spread code such as PN sequence to transmit it. When receiving such a spread spectrum signal, the spread spectrum reception apparatus at a reception side as follows. That is, it calculates a correlation value between a received spread spectrum signal and a predetermined spread code to detect a synchronization phase of spread code. Thereafter, it produces an inverse-spread of the received spread spectrum signal using the inverse-spread signal. Such inverse spread signal is then information-modulated to obtain the information signal.
During mobile communication, a portion of a transmission signal is reflected, diffracted, and scattered by topography or construction such as buildings to reach a reception side via different routes and at different times. For example, a reflected wave, which is the one that is reflected by construction, reaches the reception side later than a direct transmission wave, which is the one that directly reaching from the transmission side to the reception side. The reason behind this is that, the route of the reflection wave is longer than that of the direct wave. Time difference between when the direct wave reaches the reception side and when the delay wave, that is the reflection wave that has been delayed, will be substantially about a few 10 microseconds. The route of the signal traveling from a transmission side to a reception side is referred as “path”. Transmission environment wherein a transmission signal travels via a plurality of paths is referred as “multipath”. Under multipath environment, a reception side is destined to receive a multiplex wave. In a multiplex wave, signals of a plurality of paths of which delay times are different are overlapped because a plurality of same spread spectrum signals respectively reach at different times. In mobile communication, there is a motion on the transmission side or the reception side. Therefore, during mobile communication, fading, which is an alternation in amplification of multiplex wave, occurs. The fading occurs because a way in which the phrases are combined always changes due to the motion of the transmission side or the reception side.
A RAKE-receiver is the one which RAKE-combines (maximum-ratio-combines) a signal output from a plurality of RAKE-fingers for separating five predetermined path signals by inverse-spreading a received multiplex wave with a combiner to perform a weight application corresponding to a reception signal level. Signal-electric-power-ratio against thermal noise and interference of a received multipath signal is improved by performing RAKE reception, so that diversity reception is realized. However, it is necessary to select a plurality of path signals proper for RAKE-combination in order to inverse-spread each path signal from a multiplex wave to perform RAKE-combination.
The selection of path signals proper for RAKE-combination is performed with a delay profile indicating a correlation value calculated with a reception spread spectrum signal and a predetermined reference spread code and a delay time therein by each sample point. It is considered that an information signal is included in a signal whose correlation electric power value is greater among sampling points of the delay profile. Therefore, it will be better to select a signal of the sampling points at which the correlation electric power value is greater as a signal of the path proper for RAKE-combination. For example, in a case of a spread spectrum receiver provided with three sets of RAKE-fingers for inverse-spreading, there will be three RAKE-combinable paths. In this case, as shown in FIG. 18, a method for selecting a signal of path of detecting three sampling points in order having highest correlation value may be adopted.
If there is no time-correlation in thermal noise and interference among the sampling points shown in FIG. 18, then a sampling point is detected in order by a point whose correlation value is greater and thereafter inverse-spread signals are RAKE-combined respectively corresponding to a delay time of the detected sampling points. Thus, signal electric power Sc standardized with interference and thermal noise after RAKE-combination becomes maximum and may be represented by following equation. Si is directed to a correlation electric power value in the i-th order detection path timing.   Sc  =            ∑              i        =        1            3        ⁢                   ⁢    Si  
However, there exists a time correlation in thermal noise and interference among real sampling points. Therefore, if signals of path detected in order from a greater correlation value are merely combined, a signal electric power Sc will be reduced. The amplitude of signal electric power Sc is concretely represented by following equation. Herein, s=(s1, s2, s3) T and si is directed to a correlation value corresponding to timing i.       Sc    =                            (                                    s              T                        ⁢            s                    )                2                              s          T                ⁢        Rs                  R    =          [                                                  ρ1              ⁢              .1                                                          ρ1              ⁢              .2                                                          ρ1              ⁢              .3                                                                          ρ2              ⁢              .1                                                          ρ2              ⁢              .2                                                          ρ2              ⁢              .3                                                                          ρ3              ⁢              .1                                                          ρ3              ⁢              .2                                                          ρ3              ⁢              .3                                          ]      
ρij is directed to time correlation coefficient of noise and interference between timings i and j. Namely, the narrower, a distance between sampling points to be detected is, in other word, when arrival times are extraordinarily close (when delay times are mutually closed in a delay profile), the greater, time correlation in thermal noise and interference among these signals is. There exists a method for detecting a sampling point in order from the sampling point whose correlation value is greater by sampling points whose delay time is sufficiently spaced with respect to a detected sampling point as shown in FIG. 19 in order to eliminate affection owing to time correlation of thermal noise and interference among the sampling points and make a signal electric power standardized with interference and thermal noise.
Further, in Japanese Patent Laid-open Application No. 10-336072, a first path is detected by selecting a sampling point whose correlation value is greatest in a delay profile shown in FIG. 20 (refer to FIG. 20(a)). Thereafter, sampling points positioned within ±k numbers of sampling points (k is a natural number) are eliminated from an object to be selected with respect to sampling points having been already detected and a second path is detected (refer to FIG. 20(b)). Next, a third path is detected by selecting a sampling point whose correlation value is greatest from among sampling points positioned except for within ±k numbers of sampling points with respect to a sampling point of a second path (refer to FIG. 20 (c)). As above-explained, there exists a method for selecting a path proper for RAKE-combination by setting a distance between samples to be selected as a distance corresponding to k or greater numbers of samples.
Furthermore, in Japanese Patent Laid-open Application No. 10-308688, a delay profile is produced by performing cyclic integration after electric power conversion in order to perform average calculation by excluding affection of fading alternation and a carrier frequency deviation at the time of transmission and reception. Then, a theoretical value of result of inverse-spread of a reference code and an ideal reception signal and a portion where maximum amplification portion of a correlation value is eliminated by a pseudo correlation elimination portion is called from a delay profile to have the delay profile impulse-shaped whereby a path for RAKE-combination is detected. Additionally, a theoretical value of result of inverse-spread of a reference code and an ideal reception signal with a matrix calculation of a reception signal whereby a path for RAKE-combination is detected.
As above-explained, the reception side is destined to receive a multiplex wave wherein signals whose delay times are different have been overlapped under a multipath environment, because transmitted spread spectrum signals reach via a plurality of paths and at different times. Therefore, in order to eliminate affection of multi-fading, numbers of signals corresponding to numbers of RAKE fingers are selected by a delay profile produced by calculating a correlation value between a predetermined reference spread code and a reception multiplex signal to separate a path signal by inverse-spreading a multiplex wave in accordance with a delay time of a selected path signal to RAKE-combine the separated path signals whereby a ratio of a signal electric power to interference and thermal noise must be improved. As a result, it is important how a path signal proper for RAKE-combination should be selected in order to optimize effect of improvement of the ratio relating to the signal electric power with RAKE-combination.
For example, there exists little effect of improvement of the ratio relating to the signal electric power with RAKE-combination when time correlation of thermal noise and interference between selected paths is too great. As above, effect of improvement of the ratio relating to the signal electric power with RAKE-combination is largely dependent on correlation of thermal noise and interference of each of path signals. Because numbers of RAKE-fingers forming a RAKE-combination modulator are limited, a path signal whose time correlation of thermal noise and interference are great is not RAKE-combined, rather, a path signal whose time correlation of thermal noise and interference are great is RAKE-combined, so that the effect of RAKE-combination by the latter becomes larger than an effect when path signals are merely combined in order from a signal having greater correlation value.
According to the technology disclosed in Japanese Patent Laid-open Application No. 10-336072, sampling points positioned within ±k numbers of sampling points are eliminated from an object to be selected for a second path with respect to sampling points detected as a first path, so that there lies a problem that even if a sampling point is one capable of improving characteristic with RAKE-combination, the sampling point may not be detected as a second path. There lies a possibility that characteristic is deteriorated by RAKE-combination, because a sampling point positioned except for within ±k numbers of sampling points with respect to sampling points detected as a first path, as a second path if a correlation value is greater in spite of time correlation of thermal noise and interference. Namely, there lies a possibility that signal characteristic is further deteriorated if inverse-spread signals are RAKE-combined corresponding to a delay time of signal of each of detected paths, because the paths are not detected, considering over time correlation of thermal noise and interference.
According to the technology disclosed in Japanese Patent Laid-open No. 10-308688, a delay profile is produced by performing cyclic integration after electric power conversion. However, affection owing to electrical power conversion is not considered for this delay profile, so that any optimum path may not be detected for RAKE-combination. According to the conventional invention disclosed in Japanese Patent Laid-open Application No. 10-308688, a delay profile is made to be impulse-shaped and thereafter a path to be RAKE-combined is detected. However, even a path such as may improve characteristic by RAKE-combination is destined to be cut off. Further, in a case of even a path such as being deteriorated regarding as characteristic, if an electric power is greater, then the path is destined to be detected. Furthermore, a cyclic addition is performed at a voltage level and a delay profile is corrected at a voltage level. However, the correction may not be performed because a ratio of a signal to interference of the delay profile is too poor.
When a spread code whose period is longer than a symbol period is used, or when length of already-known transmission symbol sequence is longer, amount of calculation for obtaining a theoretical value of inverse-spread result of reference code and a reception signal becomes vast. This will result in a larger circuit which consumes higher electrical power, because, such a vast number of calculations are performed by each time of detection of RAKE-combination path timing. For example., if a cell radius exceeds over about 10 km and a chip-rate exceeds over about 4 MHz, then it must be considered that the delay spread is represented by about 256 chips. Further, if the operation is performed with 4-times-over sample, then a 1024×1024 matrix must be inversed. This is not realistic, because calculation is too vast to detect a signal of path proper for RAKE-combination, following an environment of propagation of mobile station moving with high-speed. Optionally, there is no guarantee that such an inverse-matrix always exists. As a result, there is possibility that the inverse-matrix may not exist, so that a RAKE-combination timing may not be detected.
According to the technology disclosed in Japanese Patent Application No. 10-308688, a pseudo correlation elimination portion for correcting a delay profile is arranged independently of a synchronization detection portion for detecting a path for RAKE-combination. As a result, hardware become large scaled and it consumes higher electric power.
The present invention is devised in order to overcome the above-explained problems. It is an object of this invention to select a path signal proper for RAKE-combination, considering over a time correlation of thermal noise and interference to RAKE-combine the selected path signals whereby a spread spectrum reception apparatus for improving a ratio of a signal electric power to interference and thermal noise is provided.